The present invention relates generally to antennas, and more particularly to broadband convex ground planes for multipath rejection.
Multipath reception is a major source of positioning errors in global navigation satellite systems (GNSSs). Multipath reception refers to the reception by a navigation receiver of signal replicas caused by reflections from the receiver environment. The signals received by the antenna in the receiver are a combination of the line-of-sight (“true”) signal and multipath signals reflected from the underlying ground surface and surrounding objects and obstacles. Multipath reception adversely affects the operation of the entire navigation system. To mitigate multipath reception, the receiving antenna is commonly mounted onto a ground plane. Various types of ground planes are used in practice; for example, flat metal ground planes and choke rings.
A flat metal ground plane is advantageous because of its simple design, but it requires a relatively large size (up to a few wavelengths of the received signal) to efficiently mitigate reflected signals. The relatively large size limits the usage of flat ground planes, since many applications call for compact receivers. At smaller dimensions, a choke ring mitigates multipath reception significantly better than a flat ground plane. Basics of the choke ring design are presented, for example, in J. M. Tranquilla, J. P. Carr, and H. M. Al-Rizzo, “Analysis of a Choke Ring Groundplane for Multipath Control in Global Positioning System (GPS) Applications”, Proc. IEEE AP, vol. AP-42, No. 7, pp. 905-911, July 1994. A choke ring is designed with a number of concentric grooves machined in a flat metal body. A primary application for choke-ring antennas is to provide good protection against multipath signals reflected from underlying terrain.
Common choke-ring antennas, however, have a number of disadvantages. A choke-ring ground plane contributes to undesirable narrowing of the antenna directivity pattern. Narrowing the antenna directivity pattern results in poorer tracking capability for satellites with low elevations. Also, the performance of a choke-ring structure is frequency-dependent. In a choke ring, the depth of the grooves should be slightly greater than, but still close to, a quarter of the carrier wavelength. Because new GNSS signal bands (such as GPS L5, GLONASS L3, and GALILEO E6 and E5) are being introduced, the overall frequency spectrum of GNSS signals is increasing significantly; consequently, traditional choke ring capabilities are becoming limited.
U.S. Pat. No. 6,278,407, for example, discusses a choke-ring ground plane with a number of grooves in which there are apertures with micropatch filters. The filters are adjusted such that the apertures pass low-frequency band signals (for example, GPS/GLONASS L2) and reflect high-frequency band signals (for example, GPS/GLONASS L1). The position of the apertures is selected such that it provides the best multipath rejection within the L1 band. This structure is a dual-frequency unit and does not provide good multipath mitigation within the entire GNSS frequency range. As mentioned above, the directivity pattern is also narrowed.
What is needed is a ground plane design for an antenna system with wide directivity pattern, high multipath rejection, and a broad frequency range. Efficient usage of the space inside the antenna system to accommodate various components such as a navigation receiver is advantageous.